Optoelectronic substrate and methods of making same

ABSTRACT

A method of fabricating a device by providing an auxiliary substrate having a metal nitride layer disposed thereon where the nitride layer has a nitrogen face and an opposite face and a dislocation density that is less than about 10 6 , with the nitrogen face of the nitride layer facing the auxiliary substrate; depositing at least one epitaxial nitride layer on the exposed opposite face of the nitride layer of the structure; depositing a further metal layer over at least a portion of the epitaxial nitride layer(s); bonding a final substrate on the deposited metal layer; and removing the auxiliary substrate to form the device from the final substrate and deposited layers. Preferably, the device that is formed includes a LED or laser.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of application Ser. No. 12/424,868 filedApr. 16, 2009, which is a division of application Ser. No. 11/084,747filed Mar. 21, 2005 now U.S. Pat. No. 7,537,949.

FIELD OF THE INVENTION

Articles including an optoelectronic substrate having at least oneactive nitride layer on a final substrate and a metallic intermediatelayer therebetween. These articles are made by preparing an auxiliarysubstrate wherein one semi-conducting nitride layer is placed on anauxiliary substrate; metallizing the auxiliary substrate on the nitridelayer; bonding the metallized nitride layer and auxiliary substrate witha final substrate; and then removing the auxiliary substrate to obtain anitride layer and subjacent metallized layer upon the final substrate.

BACKGROUND

In the field of optical applications, GaN-based light emitting devicesincluding light emitting diodes (LEDs) and laser diodes have attractedgreat attention in recent years because these devices are capable ofgenerating short wavelength emissions in UV and blue regions, which canhave many practical applications such as high density storage, highspeed data processing, solid state lighting, flat panel color display,and quantum computing. The realization of GaN-based layers, however, isrelatively recent in comparison to GaAs-based layers. Therefore, thetechnology of GaN-based layers is still in the development stage, andmany technical issues remain to be addressed and resolved before thoseapplications can be realized.

Considering the state of the art, it is known to produce GaN-on-sapphiretemplates for blue LED mass production. In a first step of aconventional approach, a GaN nucleation layer is grown on a sapphiresubstrate. In a second step, a two to four microns thick GaN bufferlayer is grown on the nucleation layer. This growth step is verytime-consuming and takes typically from two to four hours. In a laststep, an InGaN/AlGaN/GaN-LED structure including cladding layers,multiple quantum valves and p-type layers with a total thickness of theLED structure of about 1 μm is grown on the GaN buffer layer.

Despite the fact that a high device yield can be achieved with suchconventional technology, the resulting structures have somedisadvantages. While the sapphire substrate is less expensive, and amore popular choice than a high cost GaN-substrate, it isnon-conductive, requiring two wire bonds on top of each chip. With theelectrical current travelling laterally between these two contacts, thepackaging efficiency is greatly reduced. While sapphire is transparent,enabling more light to escape from the chip, it unfortunately acts as athermal insulator that traps heat, dramatically reducing the highoperating current efficiency and ultimately limiting the availableapplications.

Furthermore, due to the lattice mismatch and temperature expansionco-efficient difference between sapphire and GaN, the GaN devicestructures grown on a sapphire substrate are known to have many defectsthat tend to affect the device performance. Other factors, such as theinsulating property and non-cleavage of sapphire material, makemanufacture of a GaN light emitting device with such conventionalprocess technology difficult.

Instead of sapphire substrates, SiC substrates can be used to growthereon a GaN-layer. However, although conductive, SiC traps asubstantial portion of the light being emitted because massiveabsorption occurs only in the UV range.

Therefore, in another known approach for producing vertical GaN-LEDs,under consideration of the above-mentioned advantages and disadvantagesof sapphire and SiC substrates, a sapphire substrate can be used as theinitial GaN growth substrate followed by bonding a thermally andelectrically conductive metal layer on top of the GaN. By then employingan appropriate lift-off technique, the sapphire substrate is lifted offthe GaN, leaving it and the reflective base ready for the fabrication ofvertical devices.

The result of a vertical device being bonded to a reflective metal layerthat exhibits low thermal resistance, and high electrical conductivity,leads to efficient devices that lend themselves to thinner LED packagingwhile remaining rugged enough to retain comfortability with traditionaldie-mount techniques. Due to a high brightness, this approach isespecially advantageous for back light applications such as cellularphones, where a thinner die saves precious space, as well as for highpower/super bright applications, such as solid-state white lighting.

Nevertheless, even this approach cannot prevent or avoid thedisadvantages arising out of the difference of material propertiesbetween the sapphire substrate and the GaN-layer grown thereon. Inparticular, the dislocation density of the active nitride layer(s) ofsuch substrates, which is usually in the order of 10⁸/cm², stronglyrestrains the efficiency of optical devices fabricated with suchsubstrates.

It is therefore desired to provide a method of forming GaN-type layersin which the crystalline quality of the active nitride layer(s) can beimproved.

SUMMARY OF THE INVENTION

The present invention encompasses a method of producing anoptoelectronic substrate by detaching a thin layer from asemi-conducting nitride substrate and transferring it to an auxiliarysubstrate to provide at least one semi-conducting nitride layer thereon,metallizing at least a portion of the surface of the auxiliary substratethat includes the transferred nitride layer, bonding to a finalsubstrate the metallized surface portion of the transferred nitratelayer of the auxiliary substrate, and removing the auxiliary substrateto provide an optoelectronic substrate comprising a semi-conductingnitride surface layer over a subjacent metallized portion and asupporting final substrate.

In particular, the invention relates to a method of fabricating a deviceby providing an auxiliary substrate having a metal nitride layerdisposed thereon where the nitride layer has a nitrogen face and anopposite face and a dislocation density that is less than about 10⁶,with the nitrogen face of the nitride layer facing the auxiliarysubstrate; depositing at least one epitaxial nitride layer on theexposed opposite face of the nitride layer of the structure; depositinga further metal layer over at least a portion of the epitaxial nitridelayer(s); bonding a final substrate on the deposited metal layer; andremoving the auxiliary substrate to form the device from the finalsubstrate and deposited layers. Preferably, the device that is formedincludes a LED or laser.

In this method, the nitrogen face of the nitride layer is prepared tohave a surface roughness of less than 0.3 nm RMS as measured with anatomic force microscope over a field of 1×1 μm², while the opposite faceof the nitride layer has a roughness on the order of a few Angstroms RMSas measured with an atomic force microscope over a field of 1×1 μm². Thedeposited epitaxial layers are preferably n type GaN, AlGaN, InGaN, or ntype GaN having a thickness of between 0.1 to 1 micron and morepreferably about 0.5 micron. The opposite face of the nitride layer ispreferably a metal face of a Group III element such as GaN or AlN. Also,the auxiliary substrate typically comprises at least one of silicon,GaAs, or ZnO.

The invention also encompasses an optoelectronic substrate that includesa substrate, a metallized layer disposed over at least a portion of thesubstrate, and at least one nitride layer disposed over the metallizedlayer, wherein the dislocation density of the nitride layer(s) is lessthan about 10⁸/cm².

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantageous features of the present invention will become moreapparent with the following detailed description when taken withreference to the accompanying drawings in which:

FIG. 1 schematically shows a step of providing a massive semi-conductivenitride substrate according to an embodiment of the present invention;

FIG. 2 schematically shows a deposition step of a dielectric layer onthe substrate of FIG. 1 according to an embodiment of the presentinvention;

FIG. 3 schematically shows an implantation step in the structure of FIG.2 according to an embodiment of the present invention;

FIG. 4 schematically shows a bonding step of the structure of FIG. 3with an auxiliary substrate according to an embodiment of the presentinvention;

FIG. 5 schematically shows a splitting step of the structure of FIG. 4according to an embodiment of the present invention;

FIG. 6 schematically shows a polishing step of a split structure of FIG.5 according to an embodiment of the present invention;

FIG. 7 schematically shows a deposition step of a protective layer onthe structure of FIG. 6 according to an embodiment of the presentinvention;

FIG. 8 schematically shows an annealing step of the structure of FIG. 7according to an embodiment of the present invention;

FIG. 9 schematically shows a removal step of the protective layer of thestructure of FIG. 7 after the annealing step of FIG. 8 according to anembodiment of the present invention;

FIG. 10 schematically shows a growth step of an epitaxial layer on thestructure of FIG. 9 according to an embodiment of the present invention;

FIG. 11 schematically shows a deposition step of a metal layer on thestructure of FIG. 10 according to an embodiment of the presentinvention;

FIG. 12 schematically shows a final substrate with a reflection layeraccording to an embodiment of the present invention;

FIG. 13 schematically shows a bonding step between the structures ofFIGS. 11 and 12 according to an embodiment of the present invention;

FIG. 14 schematically shows the structure of FIG. 13 after a removalstep of the auxiliary substrate and the dielectric layer according to anembodiment of the present invention;

FIG. 15 schematically shows the structure of FIG. 14 after the removalof a semi-conducting nitride layer according to an embodiment of thepresent invention; and

FIG. 16 schematically shows a preparation of an electrical contact onthe structure of FIG. 15 according to an embodiment of the presentinvention.

It should be noted that the dimensions shown in the Figures are not trueto scale.

DETAILED DESCRIPTION OF THE INVENTION

While the present invention is described with reference to theembodiments as illustrated in the following detailed description as wellas in the drawings, it should be understood that the following detaileddescription as well as the drawings are not intended to limit thepresent invention to the particular illustrative embodiments disclosed,but rather to describe the illustrative embodiments merely exemplifyingthe various aspects of the present invention, the scope of which isdefined by the appended claims.

The present invention advantageously can achieve such nitride layers bypreparing the auxiliary substrate through detaching a part from amassive semi-conducting nitride substrate; and transferring said partonto the auxiliary substrate to form the semi-conducting nitride layerthereon.

With the inventive method, substrates for optoelectronic applicationshaving active layer(s) with a low density of crystalline defects can befabricated. In particular, the dislocation density of the activelayer(s) can be brought to below about 10⁸/cm² although the activelayer(s) can be made with a low thickness. Thus, optoelectronic devicesfabricated by using the inventive substrates and methods can achieve ahigh efficiency and lifetime at low dimensions and weight.

Furthermore, the intermediate metal layer can facilitate suitable orincreased electrical conduction between the active nitride layer(s) andthe auxiliary carrier substrate and can serve additionally as a thermaldrain between the active layer(s) and the auxiliary substrate. This way,the final substrate can be electrically well-contacted and thermalstress during operation can be minimized to a low value.

According to a beneficial embodiment of the present invention, thesemi-conducting nitride substrate is a GaN-substrate or an AINsubstrate. GaN and AIN have the advantage that the growth of thesematerials, as well as their properties, are relatively well known sothat these materials can be provided with a high crystal quality.

For example, the massive semi-conducting nitride substrate can beproduced with a dislocation density of less than about 10⁶/cm². The term“massive” preferably means a sufficiently large block or other mass ofnitride substrate that multiple portions can be removed to form multiplelayers according to the invention. Because of the very low dislocationdensity of the semi-conducting nitride substrate, from which thesemi-conducting nitride layer is detached, the nitride layer formed onthe auxiliary substrate can also be provided with a very low dislocationdensity resulting in a low dislocation density of layer(s) grownsubsequently on the transferred semi-conducting nitride layer accordingto the invention. With this method, the active part of the resultingsubstrate has a very good crystallinity independent from the propertiesof the final substrate.

By way of another example, the detaching and transferring comprises:depositing a dielectric layer on the semi-conducting nitride substrate,implanting atomic species through the dielectric layer into a certaindepth of the nitride substrate to form therein a predetermined splittingarea, which is also referred to herein a weakened region or weakenedzone, bonding of the nitride substrate on the implanted side with theauxiliary substrate; and thermal and/or mechanical treating of thenitride substrate to split, or weaken, said substrate along thepredetermined splitting area. These steps are taken from the knownSMART-CUT® process technology and lead to a smooth and highly accuratetransfer of a high quality nitride layer with a defined thickness ontothe auxiliary substrate.

In an advantageous embodiment of the present invention, the auxiliarysubstrate is a substrate that includes silicon, GaAs or ZnO. Thesesubstrates can provide a high mechanical strength that is explicitlyfavorable during the detaching and transferring in which the auxiliarysubstrate is under relatively high mechanical stress. Furthermore, thecoefficient of thermal dilation of GaAs and ZnO is slightly higher thanthe coefficient of thermal dilation of typical semi-conducting nitridelayers such as GaN or AIN, resulting in an active layer of the finalsubstrate having only slight compression that inhibits or preventsoccurrence of cracking effects in the active layer.

According to another beneficial variant of the present invention, theauxiliary substrate is annealed after the detaching and transferring.The annealing step reinforces connection at the interface between thetransferred nitride layer and the auxiliary substrate.

In a further example, a protective layer is brought onto the transferrednitride layer before the annealing and is removed thereafter. This way,the transferred nitride layer can be protected from chemical influencesin the annealing environment which could otherwise lead to chemicalreaction(s) with the nitride layer or to other unintentional changes ofcrystallinity or purity of the nitride layer.

In another preferred embodiment, the surface of the transferred nitridelayer is smoothed after the detaching and transferring or after theannealing. The removal of a certain degree of roughness of thetransferred nitride layer surface facilitates usefulness in formingsubsequent layers that can be better deposited on a smooth subsurface.This smoothing can be implemented before or after the annealing.

It is furthermore advantageous to deposit at least one epitaxial nitridelayer of a material of a group including N-doped GaN, InGaN, AlGaN,undoped GaN and P-doped GaN on the transferred nitride layer of theauxiliary substrate. The additional epitaxial nitride layer(s) are wellsuited to form the active layer(s) of an optoelectronic structure.

In a preferred embodiment, the metallic intermediate layer is depositedon the at least one epitaxial nitride layer. This way, the metallicintermediate layer can form an Ohmic contact for the at least oneepitaxial nitride layer. Depending on the metal species chosen to formthe intermediate layer, it could be useful to anneal to create an alloybetween the nitride material and the metals, which allows a shift from aSchottky to an Ohmic contact behavior. The transferred thin layer can beremoved from the nitride substrate after removing the auxiliarysubstrate to expose the epitaxial nitride layer.

In another example, the method further includes providing a finalsubstrate and bonding of the final substrate on the metallized side ofthe auxiliary substrate. The final substrate provides a good mechanicalsupport for the transferred nitride layer and the deposited at least oneepitaxial nitride layer from the side opposite to the auxiliarysubstrate.

In a beneficial variant, the material of the final substrate includes atleast one of silicon, silicon carbide, or copper. These materialsprovide good electrical and thermal conductivity, which is particularlyrelevant for a later optoelectronic application of the producedsubstrate wherein the electrical conductivity can be used to form anOhmic contact on the final substrate and the thermal conductivity servesto provide a good thermal drainage for an optoelectronic device with thefinal substrate.

In a yet further advantageous embodiment, at least one reflection layeris deposited onto the final substrate before the bonding step of thefinal substrate with the auxiliary substrate. The reflection layerserves as a mirror between the active layer(s) and the final substrateso that the light emitted from the active layer(s) will not be absorbedby the final substrate.

According to another embodiment, the auxiliary substrate is removedmechanically and/or chemically after the bonding, wherein the nitridelayer is used as a stop layer for the removal step. In this step, theactive layer(s) of the final substrate can be excavated.

In yet another advantageous embodiment, the transferred nitride layercan be removed from the substrate after the removal step of theauxiliary substrate. The removal of non-essential layer(s), such as thetransferred nitride layer, improves the efficiency of the wholestructure since inutile layer(s) would lead to an unwanted absorption ofphotons emitted from the active layer(s).

According to the present invention, an efficient method of producingsubstrates for optoelectronic applications, referred to herein asoptoelectronic substrates, is provided that may be used to fabricateoptoelectronic devices, such as LED structures or laser diodes. FIGS. 1to 16 show an illustrative process flow of an embodiment of the presentinvention, which includes many optional but preferred steps according tothe invention.

With reference to FIG. 1, a step of providing a massive semi-conductingnitride substrate is shown. In the embodiment shown, the massivesemi-conducting nitride substrate is a GaN-substrate 8 having a nitrogenface 18 on top and on its bottom a gallium face 19. The massiveGaN-substrate has a hexagonal crystal structure with a dislocationdensity of lower than 10⁶/cm². The planarity of the substrate 8 is inthe range of 20 μm. The nitride substrate 8 has a thickness of about 150to 750 μm. The nitrogen face 18 of the nitride substrate 8 is polishedand preferably has a surface roughness of lower than 0.3 nm RMS measuredwith an atomic force microscope (AFM) over a field of some 1×1 μm².

The above-described technology can also be realized using asemi-conducting nitride substrate of GaN with a cubic crystal structureor with a cubic or hexagonal monocrystalline AlN substrate instead ofthe hexagonal GaN-substrate 8. In all of these cases, however, thedislocation density of the substrate 8 should be from about 10⁵ to10⁶/cm² or even lower.

FIG. 2 schematically shows a deposition step of a dielectric layer 9 onthe massive semi-conducting nitride substrate 8. This deposition step istypically performed on the nitrogen face 18 of the nitride substrate 8.Preferably, a dielectric layer is provided on the semi-conductingnitride substrate prior to detaching the thin layer and dielectric layerand transferring them to the auxiliary substrate. The dielectric layer 9can be a material that includes silicon dioxide, silicon nitride, or acombination of these materials or other dielectric materials that willhave sufficient adhesion to the nitrogen face 18 of the GaN-substrate 8to remain deposited thereon. More than one dielectric layer (not shown)can be provided in successive fashion if desired, and each layer caninclude the same or different material(s) compared to the otherdielectric layers. The dielectric layer(s) 9 are preferably deposited bychemical vapor deposition, although any other deposition methodavailable to those of ordinary skill in the art may be used. Althoughnot absolutely necessary, the structure shown in FIG. 2 can beoptionally, but preferably, thermally annealed to increase the densityof the dielectric layer(s) 9.

Preferably, the inventive methods involve implanting at least one atomicspecies through the dielectric layer and into the nitride substrate to acertain depth (d) therein to form a weakened region, bond the dielectriclayer to the auxiliary substrate, and detach the thin layer anddielectric layer along the weakened region for transfer to the auxiliarysubstrate.

As illustrated in FIG. 3, the structure shown in FIG. 2 is implantedwith atomic species 10. The species 10 can be of hydrogen, helium, orother suitable elements alone or in combination. In the embodimentshown, the species 10 are implanted with energies from about 20 and 200keV and with doses from about 10¹⁵ and 10¹⁸ at/cm². The atomic species10 are implanted to a pre-determined depth d of the nitride substrate 8,thereby forming a predetermined splitting area 11, at and around theimplantation depth d.

According to FIG. 4, the implanted structure of FIG. 3 is bonded on itsimplanted side with an auxiliary substrate 6. The auxiliary substrate 6is preferably a silicon substrate, GaAs substrate, or a ZnO substrate,but can also be of another suitable carrier material which hasrelatively high mechanical stability because this material will behighly stressed during a following SMART-CUT® process in which thenitride substrate 8 is split. In the case of GaAs or ZnO as auxiliarysubstrate 6, the thermal dilation coefficient of the auxiliary substrate6 is chosen or adapted in such a way that it is at least slightly higherthan the thermal dilation coefficient of GaN, resulting in a structurehaving a GaN-layer with a slight compression that inhibits or preventsthe appearance of cracking in this layer.

As shown in FIG. 5, the structure of FIG. 4 is split into two parts withthermal and/or mechanical treatment. The stress applied due to thattreatment leads to the splitting of the structure of FIG. 4 along thepredetermined splitting area 11. The splitting step results in twostructures, a residual part of the former semi-conductive nitridesubstrate 8 and an auxiliary substrate 5 consisting of the auxiliarysubstrate 6, the dielectric layer 9 and a semi-conducting nitride layer2 being a part of the former semi-conducting nitride substrate 8. Thesplit structures have split surfaces 14 and 22 with an increasedroughness after the splitting step.

With reference to FIG. 6, the auxiliary substrate 5 is preferablysmoothed in a polishing step applied on the split surface 14 of thenitride layer 2. After this polishing step, the surface roughness of theGaN-layer 2 is of an atomic level which is only several Angstroms whenmeasured with an AFM.

In the next step, shown in FIG. 7, a protective layer 13 is deposited onthe surface 14 of the GaN-layer 2. The protective layer 13 is preferablya dielectric layer. Although not shown, additional dielectric layers ofthe same or different materials could be deposited.

As shown in FIG. 8, the structure of FIG. 7 is thermally annealed inannealing equipment 20. The structure is thermally treated in atemperature region from about 500° C. to 1100° C. in a gaseousatmosphere that minimizes or avoids degradation of the crystal qualityof the GaN-layer 2. The annealing step shown in FIG. 8 can also beapplied before the polishing step shown in FIG. 6, and can also beapplied directly onto the auxiliary substrate 5 without the depositionof the protective layer 13 before optionally annealing. The thermalannealing can increase the bonding forces at the interface between theauxiliary substrate 6 and the dielectric layer 9.

As shown in FIG. 9, the protective layer 13 that can be deposited beforethe annealing step shown in FIG. 8, is later removed. Preferably, theprotective layer 13 can be removed with a chemical treatment, forinstance with HF or another suitable acid. The removal results in theauxiliary substrate 5 having a smooth and clean gallium face 14 on topof the GaN-layer 2. The GaN-layer 2 is monocrystalline with a crystalquality at least substantially equivalent to the crystal quality of themassive semi-conducting nitride substrate 8 shown in FIG. 1. The surfaceof the GaN-layer 2 is substantially free from undesired particles orother impurities. The thickness of the GaN-layer 2 is, in one preferredembodiment, about 200 nm. Other preferred thicknesses include from about150 nm to 250 nm, although others are also included in the scope of theinvention.

With reference to FIG. 10, an epitaxial nitride layer 15 can bedeposited on the gallium face 14 of the GaN-layer 2. The epitaxialnitride layer 15 can be deposited with a known epitaxy method likeMOCVD, MBE, or HYPE. The temperature applied during the epitaxialdeposition step is preferably in the range of about 700° C. to 1100° C.

For example, the epitaxial nitride layer(s) deposited in the step shownin FIG. 10 can be: of n-type GaN doped with Si and having a thickness ofabout 0.2 microns, of InGaN, of AlGaN and/or of p-type GaN doped withMg. The total thickness of the epitaxial nitride layer(s) 15 can includeany suitable thickness, typically about 0.1 microns to 1 microns, and apreferred exemplary thickness includes that of about 0.5 micron. Thecomposition of the epitaxial nitride layer(s) depends on the efficiencyand the wavelength of the optoelectronic structure that will befabricated with the resulting substrate. The dislocation density of theepitaxial nitride layer(s) 15 is nearly equivalent to the dislocationdensity of the original GaN-substrate 8, which means no more than, andpreferably lower than, about 10⁶/cm². It is generally advantageous tohave epitaxial nitride layer(s) with an increased thickness to advancecurrent propagation in the active layer(s) of the resulting structure.

In a next step, shown in FIG. 11, a metal layer 4 is deposited on theepitaxial nitride layer(s) 15, i.e., metallization according to theinvention. The metal layer 4 serves later as an Ohmic contact to contactthe resulting structure electrically. The metal layer 4 can be of Ni/Au,Pt, rhodium, or any other suitable conductive material, alloy, orcombination of metals. The metallized layer is disposed over asufficient portion of the final substrate, e.g., over the entireepitaxial nitride layer, to increase electrical contact between thefinal substrate and the at least one nitride layer. Thus, the metallayer is subadjacent the nitride layer on the final substrate.

With reference to FIG. 12, a final substrate 7 is provided on which areflection layer 17 can be optionally, but preferably, deposited. Thefinal substrate 7 serves as a support substrate that is electricallyconductive with a low electrical resistivity and a good thermalconductivity. The final substrate 7 can be of silicon, SiC, copper, orany other suitable conductive or semi-conducting material. Thereflection layer 17 can be for instance of gold, aluminium, or silver,which materials have good reflectivity. The reflection layer 17 actslater as a mirror layer arranged between the final substrate 7 and theepitaxial nitride layer(s) 15. The mirror qualities selected is or arechosen depending on the emitted wavelength(s) of the resultingstructure.

As illustrated in FIG. 13, the structures of FIGS. 11 and 12 areconnected on the metal layer 4 and the reflection layer 17 throughbonding. The bonding leads to a molecular adhesion between thestructures of FIGS. 11 and 12 to provide a contact therebetween usingmechanical pressure and a sufficient temperature. The at least onereflection layer is preferably deposited onto the final substrate beforethe bonding of the final substrate to the auxiliary substrate.

As shown in FIG. 14, in a further step the auxiliary substrate 6 and thedielectric layer 9 are removed from the bonded structure. The removalcan include mechanical lapping and/or polishing, chemical attack usingthe gallium nitride layer 2 as an etch stop layer, or any combination ofthese or other suitable removal techniques available to those ofordinary skill in the art. If the final substrate 7 is made of silicon,e.g., the removal can be realized using mechanical treatment followed bychemical treatment based on a TMAH or HF/HNO3 solution. Such a chemicalattack can be accomplished by immersing the structure in a bath of thesolution using equipment with which the structure can be rotated and inwhich the auxiliary substrate can be exposed to the chemical solution.The removal of the auxiliary substrate can also be realized by usingchemical treatment alone.

In a next step shown in FIG. 15, the gallium nitride layer 2 is removedfrom the structure shown in FIG. 14. A removal of inutile layers, suchas the non-doped GaN-layer 2, can result in an enhancement of efficiencyof the resulting structure in that such useless layers would only leadto an unfortunate and unnecessary absorption of photons. GaN absorbs,for instance, UV radiation.

With reference to FIG. 16, an electrical contact 21 can be provided onthe epitaxial nitride layer(s) 15. The resulting structure depictedincludes the final substrate 7, the reflection layer 17, the metal layer4, the epitaxial nitride layer(s) 15, and the electrical contact 21. Themetal layer 4 and the reflection layer 17 form together a metallicjunction or metallic intermediate layer between the auxiliary substrate7 and the epitaxial nitride layer(s) 15. The metallic intermediate layercan include or not include the reflection layer 17.

In the following optional but preferred steps, which are not shown, thestructure shown in FIG. 16 is further processed by using lithographicand etch steps for chip fabrication, deposition steps of dielectriclayers for preservation of the structure, and deposition steps of metallayers to obtain contacts on both sides of the structure. Finally, thefabricated structures are typically separated into chips which arepackaged.

With the inventive method, a substrate with an active layer of high goodcrystal quality having minimized dislocation density of less than about10⁸/cm², preferably less than about 10⁷/cm², and more preferably lessthan about 10⁶/cm², and with eliminated inutile layers can be realizedfor optoelectronic applications. The good crystal quality is veryimportant for a high efficiency and long life span of the structures, inparticular for LED structures of laser diodes. In a most preferredembodiment, the substrates prepared according to the invention have adislocation density of less than about 10⁶/cm², substantially equivalentto the original nitride substrate used as a starting material.Furthermore, the elimination of GaN inutile layers in the inventivemethod allows minimization of photon absorption in the active layer(s),resulting in a high efficiency of light radiation. The inventivetechnology uses the very good crystallinity of the massive nitridesubstrate in a direct transfer of a part of said substrate to the finalsubstrate. This way, the final active epitaxial nitride layer(s) can begrown directly on the high quality transferred part with the same goodcrystallinity leading to high quality final structures.

The term “about,” as used herein, should generally be understood torefer to both numbers in a range of numerals. Moreover, all numericalranges herein should be understood to include each whole integer withinthe range.

The term “substantially free,” as used herein, means less than about 5weight percent, preferably less than about 1 weight percent, and morepreferably less than about 0.5 weight percent. In a preferredembodiment, it means less than about 0.1 weight percent. “Completelyfree” or “free” of a material refers to its complete absence.

Although preferred embodiments of the invention have been described inthe foregoing description, it will be understood that the invention isnot limited to the specific embodiments disclosed herein but is capableof numerous modifications by one of ordinary skill in the art. It willbe understood that the materials used and the chemical or processingdetails may be slightly different or modified from the descriptionsherein without departing from the methods and compositions disclosed andtaught by the present invention.

What is claimed is:
 1. An improved method of fabricating a device having a Group III nitride layer, in which method an auxiliary substrate is provided having a first Group III nitride layer disposed thereon, and then at least one epitaxial Group III nitride layer is deposited on an exposed face of the first Group III nitride layer, and then a further metal layer is deposited over at least a portion of the epitaxial Group III nitride layer(s), and then a final substrate is bonded on the deposited metal layer, and finally the auxiliary substrate is removed to form the device from the final substrate and deposited layers, wherein the improvement comprises providing the auxiliary substrate by the steps comprising: preparing a bonding surface of a free-standing Group III nitride substrate by polishing the N-face of such substrate and then by depositing a dielectric layer onto the polished N-face, wherein the free-standing Group III nitride substrate has a dislocation density that is less than about 10⁶ and an exposed Ga-face opposite to the N-face; implanting one or more atomic species through the dielectric layer into the Group III nitride substrate in order to form a splitting area below the N-face of Group III nitride substrate; bonding the bonding surface of the dielectric layer of the free-standing Group III nitride substrate to the auxiliary substrate; and splitting the free-standing Group III nitride substrate at the splitting area to leave the auxiliary substrate with the first Group III nitride layer disposed thereon, the first Group III nitride layer having a nitrogen face that faces the auxiliary substrate and an opposite Ga-face that is exposed.
 2. The improved method of claim 1 wherein the improvement further comprises polishing the exposed Ga-face of the split surface of the first Group III nitride layer.
 3. The improved method of claim 2, wherein the exposed Ga-face of the metal nitride layer has a roughness on the order of a few Angstroms RMS as measured with an atomic force microscope over a field of 1×1 μm².
 4. The method of claim 3, which further comprises depositing one or more epitaxial layers upon the exposed Ga-face of the metal nitride layer.
 5. The method of claim 4, wherein each deposited epitaxial layer has a dislocation density that is less than about 10⁶.
 6. The method of claim 4, wherein each deposited epitaxial layers comprises n type GaN, AlGaN, or InGaN.
 7. The method of claim 4, wherein each epitaxial layer is deposited to a thickness of between 0.1 to 1 micron.
 8. The method of claim 4, wherein each deposited epitaxial layer has a thickness of about 0.5 micron.
 9. The method of claim 4, which further comprises providing an electrical contact on the deposited epitaxial layer(s).
 10. The improved method of claim 1 wherein the fabricated device is structured to function as a high-efficiency, long-lived optoelectronic device.
 11. The improved method of claim 1, wherein the device that is formed includes a LED or laser.
 12. The improved method of claim 1, wherein the nitrogen face of the Group III nitride substrate is polished to have a surface roughness of less than 0.3 nm RMS as measured with an atomic force microscope over a field of 1×1 μm².
 13. The method of claim 1, wherein the auxiliary substrate has a coefficient of thermal expansion that is higher than that of the metal nitride layer.
 14. The method of claim 1, which further comprises removing the Group III nitride layer after removing the auxiliary substrate.
 15. The method of claim 1, wherein the Group III nitride layer is GaN or AlN.
 16. The method of claim 1, wherein the auxiliary substrate comprises at least one of silicon, GaAs, or ZnO.
 17. The method of claim 1, which further comprises providing a reflection layer disposed upon the Ga-face of metal nitride layer of the auxiliary substrate. 